LATE JURASSIC PALEOCLIMATE OF CENTRAL AFRICA

The Samba borehole, drilled near the center of the Congo Basin, includes nearly 325 m of fluvial and lacustrine strata within the Upper Jurassic–Lower Cretaceous Stanleyville Group. These strata were deposited at ~15°S within central Gondwana and contain poorly-developed paleosols dominated by vertic features. Paleosol development is restricted to the finer-grained upper 200 m of the Stanleyville section; the lower part of the section is dominated by cross-bedded fluvial sandstones with no indication of pedogenesis. XRD analysis of the clay fraction from the paleosols reveals assemblages composed primarily of illite and mixed-layer illite-smectite, with trace amounts of kaolinite, palygorskite, and analcime.

The abrupt change from coarse-grained fluvial sands to a finer-grained mixture of lacustrine and fluvial deposits may reflect either a climatic shift toward more humid conditions or a decrease in rates of tectonic uplift in sediment source areas. Presence of palygorskite and analcime in the upper paleosol-bearing part of the Stanleyville is indicative of aridity, and vertic features in the paleosols likely result from extremely seasonal precipitation patterns. Paleoclimate during deposition of the Stanleyville Group was arid, with a small amount of rainfall occurring seasonally. These results are consistent with data from other low paleolatitude, continental, Upper Jurassic deposits in Africa and with regional projections of paleoclimate generated by general circulation models.